Why turning gear is so important on startup and stopping on steam turbine operation?

Turning gear is to ensure uniform cool down of the steam turbine’s shaft. Its occurrence protects from sagging/bending of the turbine rotor.

During Start up giving steam(Initial torque) directly can create load on turbine and can cause damage in the turbine .

During Stopping Purpose,Turbine is at a very high temperature and can cause sagging (wobbling) if not it will rub the seals and cause damage to seals and Rotor blades.